If you type /Desktop with the / before the word Desktop, for linux it means you want to go to the root directory which is / and then go to the Desktop folder. That means that the Desktop folder would not be in /home/username/Desktop but in /Desktop which would be the error here.
In your case and everybody else, the Desktop folder is normally in the /home/username/Desktop.
So if you open the terminal and you are already in your user directory, for example /home/username then you only need to just type cd Desktop because you are already in the directory where desktop is.
If you were in for example /var/www and you want to go to your desktop you would type one of the following:
cd ~/Desktop which is the same as typing /home/username/Desktop because the ~ will by default point you to the directory of your username. Think of it like ~ is equal to /home/username.
cd /home/username/Desktop
- Or doing it by steps:
cd /home, cd username, cd Desktop.. at last!
to know in what directory you are in type pwd (Print Working Directory).
Also remember that everything is case sensitive. Desktop is not the same as desktop or DeskTOP
Here are some images of what it looks like to me when I open the terminal and go to Desktop. The first image says Desktop/ because I typed "des" and then pressed TAB (which autocompletes the word and adds the trailing / symbol. The second one I typed Desktop:


Now the next 2 images are if you start from another place. In this case we are in /var/www/:

